A special type of volcano on Mars has recently been proposed to be caused by escaping gas, fluid, and mud. These 'mud volcanoes' have their counterparts on Earth. On Mars they are observed in the thousands in Acidalia Planitia in the northern Plains. Their importance is that the fluid is likely water and hence such features are more likely to harbor at least single-cell life. Here are two examples:

Among martian features believed volcanic in nature are linear ridges similar to the wrinkle ridges found in lunar maria. Here is a topographic map made from MGS MOLA measurements that includes (in the purple) these ridges and shows the diversity of other landforms.

Another example of what is interpreted as wrinkle ridges is this:

In contrast to the volcanoes described above, which are upward conical prominences, are the downward indentations or craters that can be either volcanic or impact. Both are typical of martian terrains. Extensive impact cratering was observed by Mariner 4, which sent back the first ever images taken of another planet's surface (one of these images is seen below (top) when this probe approached to within 9800 km (6086 miles). As imaged the next year by Mariner 6, the Sinus Sabeus region of the southern highlands (bottom scene) preserves typical impact craters in the ancient terrain that apparently has not been extensively resurfaced by lavas. Note that none of the larger craters in this view have central peaks.

Mariner 9 and the Vikings confirmed that a large fraction of the (older) martian surface, mainly in the southern hemisphere, remains heavily cratered. This is evident in this sketch drawing from Mutch et al., The Geology of Mars, 1976 in which all craters larger than 15 km are positioned.

A recent study made by Dr. Herbert Frey of NASA Goddard - assisted by his teen age daughter Erin - has led to a map of the distribution of large surface-visible plus now buried impact structures that nevertheless show circular surface manifestations. The latter have been located using the MOLA laser altimetry data.

One can argue that this landscape has many similarities to the still cratered Earth in its early stages before extensive water had collected into major oceans. Likewise, buried impact structures can be discerned on the lunar surface. These have since been covered by lunar ejecta. This may mean that the martian Highlands surface is also covered by ejecta deposits that spread over older craters.

Some of the martian impact structures retain well-preserved ejecta blankets that display prominent lobes, such as seen here around the crater Yuty. The ejecta was probably fluidized by vaporization of carbon dioxide-rich ice lying just beneath the surface.

One type of impact crater is different from those on the Moon, Mercury and Venus in that the edge of the ejecta blanket has a steep scarp, evident in the Viking image below, or even a peripheral rise called a rampart. This type is called a pedestal crater.

On Mars many of the younger craters still preserve their ejecta blankets, as exemplified here:

This next crater is small, young, and shows most of the same features as do terrestrial craters. Located in Terra Meridiani, this crater is 2.6 km wide (1.6 miles; rim to rim), has at least 1 nested slump zone in its interior and a distinct exterior ejecta blanket, and has exposed what appears to be internal layering of the martian surface units. The image was made by the Mars Global Surveyor.

This type of central (interior) layering, almost certainly sedimentary (see pages 19-13a and 19-13b) also appears in the 2.3 km (1.5 mile) wide Schiaparelli crater in the Chrysae Basin, seen below. The layering appears horizontal:

These observations of sedimentary-like crater interior floors and walls (layering is also discussed on the next page) seem rather mysterious. On Earth, craters that still retain their original rims (almost?) never show the bedrock below the final crater excavation wall. Yet this is common in martian craters with initial walls intact. Martian planetologists have suggested removal by erosion (they mean almost certainly wind erosion). There may be an alternate cause: the lower martian gravity allow nearly complete escape during crater formation of the bulk of the ejecta; the floor remains exposed because in the smaller craters slumping has not destroyed the walls.

Still another large impact crater, Poona, has a remarkable uniform set of rays, equispaced over the full 360° around the rim:

This small crater (below) shows a distinct pattern of dark rays. Because martian winds are continually altering the surface, both removing and covering up debris, the crater (and those above with lighter-toned rays) can be young - age estimates have ranged between a few thousand and a few million years.

This rayed crater looks fresh. Experience on Earth indicates that impacts occur rather often in terms of a human time frame. A new crater was produced on Mars during the operational period of the Mars Global Surveyor. This before-and-after image pair shows the appearance of dark rays around an area which contains a small hole not there on the earlier date:

This next Viking scene, in the southern Highlands, seems to have both impact and volcanic craters. Some without ejecta beyond their rims, especially the elliptical one, are calderas. Several others have aspects more characteristic of degraded impact structures. This was an active region, with channels (either volcanic or stream) and other types of terrain.

Now look at these three craters (Ulysses Patera):

Because of several factors, some martian craters appear as faint rings rather than topographic features raised above the surface. These have been called "ghost" or "stealth" craters. They represent some combination of burial by crater ejecta, wind erosion, dust cover, and ice cover. Here is an example of this last type:

There is evidence that the number of observed impact craters on Mars is less than would be expected if the recent activities (dust transport and deposition, ice relocation, etc.) had not buried the smaller ones. The wind, however, is capable of exhuming such craters, as displayed in this image which also shows the exposed craters to contain some signs of filling by sediment, now revealed as faint layers.

Not all impact craters are circular or slightly elliptical. Strongly elongate craters are found on the Moon. A few such distorted craters are present on Mars, such as the one shown below. The usual explanation is that the impacting body comes onto the surface at a very low or grazing angle, scouring out the surface material as it proceeds forward:

Large, young impact craters are few but conspicuous. Galle Crater is 220 km (138 miles) wide and retains its original rim:

As with the Moon, Mars has a few craters so large that they can be called impact basins. By far the biggest is the Borealis Basin (also known as Vastitus Borealis). Mars geologists (starting with George McGill and Stephen Squyres) have postulated that this feature (which has dimensions of 10,600 by 8500 km) was produced by a glancing collision with an asteroidal body that may have been as much as 1600 km in diameter. This impact, which peeled off at least 3 km of martian surface, may have occurred as early as 4 billion years ago. It accounts for the generally lower topography of the northern half of Mars (see page 19-10), evident in this topographic map (blue low; reds high):

Papers from a group at MIT in a June 2008 issue of the journal Science offer strong evidence for the existence of the Borealis Basin; as is often the case, there is a vocal group of doubters. But, the impact hypothesis is a plausible explanation for the dichotomy in martian topography: the distinctly lower top 40% is readily explained as caused by the stripping of crust from an oblique impact. In the next figure, the Borealis Basin (bluish area in upper right panel) is compared with the Hellas Basin on Mars and the Aitken Basin on the Moon:

The largest well-defined impact basin on Mars, and second in the Solar System only to the Aitken basin on the Moon, is the Hellas Basin in the southern Highlands. Its diameter is about 2100 km (1300 miles), its depth is almost 9 km (6 miles) and its rim exceeds 1.5 km (1 mile). In this view the Basin appears to have no significant landforms within it.

To emphasize the size of this structure: If all material excavated from it were to be spread evenly over the 48 continental United States, a layer of debris some 3.5 km (2 miles) thick would accrue. Below is an enlargement of the map covering this structure.

The floor of Hellas actually shows diverse landforms (mostly of low relief), some of which appear volcanic in origin; if so this would imply that the basin filled with melt soon after the impact event, which may have been relatively recent.

Another impact structure is the Argyre Basin (600 km; 390 miles diameter), seen in this Viking view: